For us here close to the alps, 10'000 ft is almost the natural habitat
. With a lot of patience and a few tricks I got my old C150 up to 13'000 ft once briefly. It's fun, the view is great (in the alps at least) and if you are going places you MAY have an advantage in range and speed.
Most has been said. Mixer issues are an item of wide and wild discussions, in the end it comes down to the simple fact that engines like optimum mixture whenever they can get it. In my 150 the O200 would let me know quickly if it was over rich, leaning it was a question of watching the RPM and listening/feeling the engine. Peak RPM is mostly sufficiently exact for these engines, EGT is more comfortable. Some flight schools have a lot of fear of leaning, unwarranted and expensive. It would be close to running your car at full choke (remember those?).
Apart, some airports here even require leaning on the ground in order to get the necessary take off power. Samedan (LSZS) is at 5600 ft AMSL and can have up to 25degrees C in Summer.
Descent is fun time, you finally get some decent speeds into these animals. 500 fpm is good for the ears and comfortable, while delivering a few knots above the usual cruise speeds, often enough the only time you'll ever see the needle near the yellow arc.
Have fun and enjoy the view.
Best regards
AN2 Driver